For a short while now, Microsoft made available 30 day trial instances for MDM as well as a short guided trial for MSL. The sign-up URLs are as follows:
Microsoft Dynamics CRM
Microsoft Dynamics Marketing (MDM)
Microsoft Social Listening (MSL) – 30 minutes guided trial
If you want MDM added to the same O365 instance along with your CRM Online, make sure you log-in with the O365 admin account where your CRM is before signing-up for MDM.